The invention relates to a power supply circuit for supplying a load from input voltages of different magnitude, comprising a first series arrangement formed by a primary winding of a transformer and a main current path of a first semiconductor switch; a second series arrangement formed by a secondary winding of the transformer and a rectifier diode, this second series arrangement having terminals for connecting the load; a second semiconductor switch for switching the first semiconductor switch; and means for generating a signal which is a measure of the current through the primary winding.
Such a circuit can, for example, be used for charging a rechargeable battery or for supplying an electronic apparatus, it being possible to connect the apparatus without switching to voltage sources of different magnitude, for example to the electric power mains in different countries. Such a power supply circuit can more specifically be used in a rechargeable electric shaver in which the circuit produces the current for charging the battery or for supplying the motor.
A power supply circuit of the type referred to in the opening paragraph is disclosed in EP-B1-0030026. This European Patent describes a power supply circuit in which during the so called, forward phase, a current flowing through the primary winding increases linearly with time. This current causes energy to be stored in the transformer. In the emitter line of a first transistor, which constitutes the first semiconductor switch, a resistor is included across which a voltage is produced which is proportional to the primary current. The base-emitter junction of a second transistor is arranged in parallel with this resistor. This transistor constitutes the second semiconductor switch. The resistor constitutes said means for generating the signal which is a measure of the primary current. At a given value of the primary current the second transistor becomes conductive and consequently the first transistor is rendered non-conducting. This terminates the forward phase and the so called flyback phase starts. Because of the energy stored in the transformer a secondary current flows in the secondary winding during the flyback phase. This current forms the charging current for the batteries or directly provides the supply current for the relevant apparatus. The secondary current decreases linearly with time. At the end of the flyback phase the first transistor is turned-on again, and another forward phase begins.
In the prior art supply circuit a signal which linearly increases with time is applied during the forward phase to the control input of the second semiconductor switch. Due to this signal, the second semiconductor switch is driven relatively slowly into the conducting state. This implies that the instant at which the second semiconductor switch becomes conductive and consequently the first semiconductor switch is rendered non-conducting is not accurately defined, so that the turn-off value of the primary current is not accurately defined.